The invention relates to a process for the abrasive machining of surfaces of semiconductors.
Chemical mechanical polishing (CMP) is frequently used in the production of large scale integrated circuits for the purpose of planarizing dielectrics or indirect structuring of wiring planes, i.e. for removing elevated areas from a structured surface. A liquid that has been mixed with polishing grains, preferably of a high hardness, and in some cases contains basic chemicals, known as a slurry, is often introduced between the surface of a semiconductor wafer that is to be machined and a polishing pad. The pad and the surface that is to be machined are in surface-to-surface contact with one another and are moved relative to one another, so that the polishing grains which move between the two surfaces abrade the surface which is to be machined.
High topography selectivity is desirable for efficient planarization of evenly structured surfaces. Therefore, elevated areas should be abraded to a greater extent than areas that lie at a lower level. This cannot be ensured under all circumstances by the slurry method, particularly when both large and very small structures occur together. The polishing grains that are entrained with the slurry are able to exert an abrading action even in the lower-lying regions, so that overall a greater amount of material than just the layer thickness of the elevated structures has to be removed for complete planarization.
Better results have recently been achieved by what is known as fixed abrasive CMP. In this process, the polishing pad is covered with a polishing device, e.g. a polishing cloth, in which the polishing grains are fixed in a polishing-grain carrier and only project above the surface of the latter in certain regions. In fixed abrasive CMP, the polishing device and the surface that is to be machined are brought into contact with one another and set in motion relative to one another. Depending on the specific device, this can be achieved by moving only one surface or by moving both surfaces. In addition, depending on the particular requirements it is possible to add suitable liquid chemicals, in order to produce chemical abrasion at the same time as the mechanical abrasion. Since the polishing grains only interact with the surface that is to be machined at the actual points of contact between the polishing devices and the surface which is to be machined, fixed abrasive CMP is able to achieve a particularly high topography selectivity.
In the strictest mechanical sense, fixed abrasive CMP is a grinding process rather than a polishing process, since the grinding or polishing grains cannot move freely, but rather are fixed randomly in a carrier, in particular at the surface of the latter. Nevertheless, in the present context the term xe2x80x9cpolishingxe2x80x9d has become the accepted word, and consequently it will also be used in the present description.
It is inevitable that an in some cases considerable number of polishing grains will become detached from the carrier during the machining operation, depending on the type of wafer and/or polishing device, so that, first, a xe2x80x9ctruexe2x80x9d polishing process also always takes place and, second, over the course of time the polishing device becomes blunt or aggressive, i.e. the amount of material removed per unit time drops or increases.
The latter phenomenon is extremely undesirable in series production, in which a large number of wafers are successively subjected to the same CMP working step, since the same, presettable parameters for a working step, such as for example machining time, chemicals selected, etc. would lead to different results depending on the degree of wear to the polishing device. Particularly as structures become ever smaller, fluctuations of this nature cannot be tolerated.
A phenomenon that has a similar result also occurs with the slurry method explained above. However, the processes that lead to the abrasive becoming blunt are different. Specifically, in the slurry method the surface of the pad, which is actually elastic, xe2x80x9cvitrifiesxe2x80x9d, i.e. its pores become blocked with the relatively small polishing grains and in particular with material that has been abraded from the surface that is to be machined. This leads to a hard and flat pad surface, leading to a considerable change in abrasion rates. This is generally counteracted by a cleaning roughening of the pad surface with the aid of a diamond needle. However, this method is too coarse for the fixed abrasive method, since it would lead to the substantially pore-free polishing-grain carrier being destroyed, and therefore it cannot be used for this method.
Therefore, the problem is currently combated by replacing the polishing device in steps in each case before machining of a new wafer. For example, certain CMP devices offer an automatic feed of polishing devices (roll to roll polisher). However, such a configuration is expensive in two respects. First, a device of this type requires considerable mechanical outlay. Second, the consumption of the polishing device is excessive. This is a significant cost factor. On account of the extremely small size of the structures that are to be machined, polishing cloth that is customarily used has to satisfy extremely high demands on accuracy both with regard to its mechanical properties and with regard to the number, size and uniformity of the polishing grains. This makes production complex and correspondingly expensive.
It is accordingly an object of the invention to provide a process for the abrasive machining of surfaces, in particular of semiconductor wafers that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which fluctuations in abrasion caused by the polishing device becoming blunt or aggressive are avoided as far as possible while the cost of replacing the polishing device is considerably reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for abrasive machining. The process includes subjecting successively a plurality of wafers having surfaces of a first type to a polishing step. During the polishing step the surfaces of the first type are in each case brought into contact with a sheet-like polishing device. The polishing device has a polishing-grain carrier with polishing grains fixed therein. The surfaces of the first type are moved relative to the polishing device, so that, as a result of an interaction between the polishing grains fixed in the polishing-grain carrier and a surface being machined, material is removed from the surface, and during the polishing step the polishing grains can become at least partially detached from the polishing-grain carrier. A conditioning step is performed for regenerating the polishing device before performing the polishing step on at least one of the wafers having the surfaces of the first type. The conditioning step includes bringing the polishing device and a conditioning surface having a strong structure into contact with one another and moved relative to one another, with a result that a regenerated state of a polishing device surface is achieved. The regenerated state is a starting state of the polishing device surface before the polishing step is performed and starting states of different polishing steps are comparable with each other at a beginning of each of a series of the polishing steps. Wafers having a surface of a second type are used to provide the conditioning surface.
Specifically, these features have the following significance. Before each individual polishing step, i.e. in each case between the machining operations carried out on the surfaces which are to be machined in succession or in each case before a sequence of individual polishing steps, an intermediate step is carried out, in which the polishing device is brought into contact with and moved relative to a special conditioning surface having a strong topography, referred to below as the xe2x80x9cdummy waferxe2x80x9d. The result is that the polishing device is regenerated, so that before the beginning of a new polishing step on a new wafer, the polishing device is in approximately the same starting state as at a beginning of a preceding polishing step carried out on the wafer that has just been machined. This ensures that identical machining times lead to identical abrasion results, so that fluctuations in series production are avoided. Therefore, the polishing device only has to be replaced after a relatively large number of individual polishing steps. Therefore, incremental values for the individual polishing steps are reduced with regard to the polishing device. However, this number can easily be determined empirically for a given type of polishing device and a given topography which is to be machined and accordingly can easily be matched to the overall series production process.
The additional costs that are generated by the additional working step can easily be more than compensated for by the savings achieved by the reduced consumption of the polishing device. In addition, a greater process stability results in that less rework is required and a higher product quality can be achieved, with less variation between the products. Particularly for the remachining of wafers that have already been substantially planarized, conditioning of the polishing device as described by the present invention is advantageous or necessary.
The invention makes use of the discovery that current fixed abrasive polishing device, for example polishing cloths, have a body in which the polishing grains are in a three-dimensional, uniform distribution. However, in each case only the polishing grains that project above the surface of the carrier material interact with the surface that is to be machined. If the polishing grains become detached from the carrier surface during the polishing operation, the surface is depleted. However, the overall three-dimensional density of the polishing grains in the carrier scarcely changes. Individual grains become detached from the carrier surface primarily at an advanced stage of the planarizing operation. At an earlier stage, i.e. while the surface that is to be machined still has a strong topography, although individual grains are torn out of the surface, this action is associated with and compensated for by abrasion of the carrier material, which leads to further grains in lower layers of the polishing device being exposed. As a result, therefore, the blunting effect described above only occurs once the structures have been substantially leveled.
If a fixed abrasive polishing device that has become blunt is, in surface-to-surface contact with a strongly structured surface, moved relative to the surface, the abrasion of the carrier material predominates at the surface on account of the lack of polishing grains that can become detached. This exposes new polishing grains, until abrasion and detachment are in equilibrium. The polishing device is then fully regenerated. The corresponding surface properties can be reliably restored by repeating the process each time the polishing device has become blunt (i.e. degraded) and polishing a new wafer with the same starting parameters as the wafer which has previously been machined.
In accordance with an added mode of the invention, the conditioning step differs from the polishing step only with regard to a choice of surface that is brought into contact with the polishing device.
In accordance with another mode of the invention, there is the step of forming the conditioning surface from a hard ceramic. The conditioning surface has a microscopic topography, a roughened surface, and surface dimensions of the microscopic topography approximately correspond to a topography of the wafers which are to be machined during the polishing step. The conditioning surface may have a topography that has a microscopic distribution.
In accordance with a further mode of the invention, during the polishing step of polishing the surfaces of the first type, the wafers having the surfaces of the second type are used as the conditioning surface.
The wafers having the surfaces of the first type are used in the production of electronic components, in particular for producing memory elements.
In accordance with a concomitant feature of the invention, during a first processing phase for polishing the surfaces of the first type, wafers having the surfaces of the second type to be polished in a second processing phase are used as the wafers having the conditioning surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for the abrasive machining of surfaces, in particular of semiconductor wafers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.